Although HLA haplotype-mismatched transplantation is an extremely promising therapeutic option, its success is challenged by many obstacles, such as leukemia relapse, toxicity of conditioning regimens, and infectious mortality, which are related to the need for extensive T cell depletion of the graft to prevent otherwise lethal graft versus host disease (GVHD). The long-term objectives of the project are to overcome these obstacles. Innovative approaches for the adoptive transfer of two types of immune cells, i.e., NK cells and T cells, will be developed and tested. Part of the project will focus on the therapeutic use of adoptively transferred donor versus recipient alloreactive NK cells in animal models and in humans. The first specific aim is to determine the sensitivity of human hematologic malignancies to human alloreactive NK cells in vivo in pre-clinical mouse models and to overcome resistance to alloreactive NK killing by means of antibody-dependent cell cytotoxicity (ADCC). Specific aim 2 will attempt to improve immune reconstitution (both peripheral expansion and thymic function) through the adoptive transfer of alloreactive NK cells in a reduced-intensity conditioning regime in murine models. Our data in mice show that alloreactive NK cell-based conditioning allows transplantation of T cell-replete MHC disparate grafts without causing GVHD and with no need for post-grafting immune suppression to control GVHD. Therefore the high number of mature T cells in the graft will not be antagonized by GVHD prophylaxis and will re-expand faster, thus improving post-transplant recovery of immune competence. Thymic function will be improved because of no thymus damage due to lethal irradiation and GVHD. Specific Aim 3 consists of a phase I clinical trial of the adoptive transfer of alloreactive NK cells in a reduced-intensity conditioning regimen for AML patients, without a matched donor, who are unable to withstand the toxicity of current high-intensity conditioning regimens. If successful, we will proceed to a phase II trial. The other part of the project, with specific aims 4 and 5 will assess whether adoptively transferred T cells with either a narrow or a broad repertoire can provide protection against infection in humans and mice. in aim 4, attempts will be made to supplement post-grafting immune competence in human transplant recipients through the adoptive transfer of aspergillus-specific T cells by means of our recently developed strategy which safely and effectively transferred donor functional pathogen (CMV)-specific immune cells across HLA barriers. Specific aim 5 will attempt to improve immune reconstitution in mice by adoptive transfer of broad repertoire T cells, with emphasis on the potential role of host T cells. The end point will be to evaluate immune reconstitution after infusion of cell doses which pose minimal risk for rejection by host T cells or GVHD by donor T cells.